Conventional dynamic random access memory (DRAM) memory module architectures typically include line termination resistors on the motherboard. Although the termination resistors on the motherboard reduce some reflections on the signal lines, they are unable to prevent reflections resulting from the stub lines that connect to the DRAM on the module. That is, termination resistors mounted on a motherboard do not prevent transmission line effects from the transmission line stubs between the resistors and the receiving device. For instance, a signal propagating from the memory controller to the DRAM encounters an impedance discontinuity at the stub leading to the DRAM on the module. The signal that propagates along the stub to the DRAM is reflected back onto the signal line, thereby introducing unwanted noise into the signal.
The introduced noise and the consequential signal degradations that are not addressed by such off-die termination become more pronounced with higher data rates and longer stub lengths. Larger, multi-drop systems containing multiple DRAM modules introduce even more reflections and consequently add more reflective noise, thereby resulting in further signal degradation.
One solution is to provide on-die termination for transmission lines, instead of line termination on the motherboard. By placing the termination resistance on the die itself rather than on the motherboard, the reflections resulting from discontinuities in the line are significantly reduced. This produces a cleaner signal and enables faster data rates.
However, problems may be presented when the transmission lines are terminated using on-die resistors. Process variations for manufacturing semiconductor devices may result in fluctuating resistance values. When implemented as an on-die termination resistance, this results in improper termination valuation. Also, temperature and voltage fluctuations presented on the die will affect the resistance values of the semiconductors used for building resistors used for termination on the die. This results in a variation of the on-die termination resistance, which increases the noise and reflections on the transmission line negatively affecting the signal transferred along the transmission line.
What is needed is an on-die termination resistance circuit that accounts for process variations in the manufacture of semiconductor devices, as well as temperature and voltage fluctuations affecting devices used for termination resistance on a die.